The purpose of this project is to study the interactions of various drugs of abuse on schedule-controlled behavior in baboons. These studies will include the analysis of drug interactions under both food-reinforced behavior and drug self-administration schedules. These studies are organized as follows: (1) The effects of drug pretreatment will be determined on self-administration of either the same compound (eg. codeine pretreatment effects on codeine self-administration) or a different compound (eg. codeine pretreatment effects on alcohol self-administration). (2) The sequential self-administration of drugs from different pharmacological classes (eg. alcohol self-administration followed by cocaine self-administration) will be studied. Food-reinforced responding will be included in these schedules to provide data on the behavioral consequences of sequential drug self-administration on responding maintained by a non-drug reinforcer. (3) The self-administration of drug combinations will be studied in another group of baboons. A multiple food and drug reinforcement schedule will be used in order to determine the behavioral consequences of drug combination self-administration. (4) The effects of sequential administration and combined drug administration of food-reinforced responding alone will be determined in a separate groups of baboons trained to respond under a multiple fixed interval 5 min fixed ratio 30 schedule of reinforcement.